Observation of phase transitions in cavity magnon-polaritons*

Hybridizing collective spin excitations in ferromagnetic crystals and a cavity with high cooperativity provides a new research subject in the field of cavity quantum electrodynamics. Owing to the damping of both photons and magnons, the polaritons have limited lifetimes. However, stationary magnon-polariton states can be reached by a dynamical balance between pumping and losses, so the intrinsical nonequilibrium system may be described by a non-Hermitian Hamiltonian. We designed a tunable cavity magnon-polaritons system which has non-Hermitian spectral degeneracies. By tuning the magnon-photon coupling strength, we observed an exceptional point and spontaneous symmetry-breaking in the cavity magnon-polariton system, where coherent perfect absorption is achieved in the unbroken-symmetry regime but not in the broken-symmetry regime. Meanwhile, the experimental results clearly display a phase transition of the system from the magnetically induced transparency regime to the weak-coupling regime.